I believe this saying is true, to an extent. I believe we all have evil "in us" to an extent. We even experience it. It's called an Intrusive Thought. It's when we have an involuntary unwelcome thought. These are usually depressing, upsetting, and difficult to get out of you mind. These thoughts are just brushed off by most people. HOWEVER, we still have these thoughts, which makes me think evil is in all of us. The difference is that most of us don't act on it.